In the pulp and paper industry, economic and environmental factors necessitate recycle of pulping chemicals and recovery of value from the organic residues of pulping. These residues are not traditionally useful for fiber making. Also, it would be environmentally and economically useful to convert other forms of biomass, e.g., wood, municipal and agricultural wastes, energy crops, etc., to premium products. It is useful to recover metals, metal oxides, and metal carbides from any type of biomass that contains metals or other precursors to these substances. In pulping processes, for example, it is desirable to recover from spent pulping liquors compounds that are equivalent to, or closely chemically similar to, spent or partially spent pulping chemicals.
Current technologies use thermal or biological means to convert biomass to energy, e.g., by direct burning or gasification, sometimes with further combustion of the resulting gas to generate mechanical energy or electric power. Thermal and biological means are also used to convert biomass to chemicals or chemical feedstocks. An example is gasification of biomass to form synthesis gas, i.e., mixtures of CO and H2 that, by means of catalytic processing, can be converted to a wide range of fuels and chemicals. Biomass is also being used by at least one automobile manufacturer to fabricate body parts for busses.
Plasma gasification of black liquor has been extensively studied, but lower operating temperatures were utilized and different results were obtained. Few of these technologies, other than niche markets such as energy recovery in the forest products industry have seen substantial, economically successful commercialization.
Chemical treatment of wood and other biomass is the dominant technology now used to produce pulp, i.e. cellulose or cellulose-rich material, for subsequent conversion to paper and paper products. Chemical treatment typically involves digestion of sized pieces of wood etc. in basic (Kraft process) or acidic (sulfite process) liquors at temperatures of 120 to 180° C. for times of 0.5 to 14 hours. By-products of this treatment are so-called waste liquors, which are aqueous suspensions of spent or partially-spent inorganic pulping chemicals and various organic residues that include lignin and lignin-derived compounds. To control costs and avoid environmental damage, it is essential to recycle the pulping chemicals and recover value from the organic wastes.
At present, some premium substances are recovered from organic residues of pulping, but generally at modest scale. For example, the U.S. pulp and paper industry recovered about 30 million gallons per year (c. 1995) of turpentine (a volatile mixture of monoterpenes), which is distilled from various pine woods at temperatures≦132° C. during heating in the pulping digester. Tall oil, a by-product of saponified fatty acids (30-60%), resin acids (40-60%, including mostly abietic and pimaric acids), and unsaponifiables (5-10%) are obtained from waste liquors produced in Kraft processing of softwoods. Organic wastes from sulfite pulping remit hexoses, which are fermented to ethanol, and lignosulfonate salts, which are used in leather tanning, ore floatation, resins, drilling mud dispersants, and manufacture of artificial vanilla. These recoveries account for relatively small fractions of the organic wastes. Thus, in 1995, the U.S. pulp and paper industry recovered about 450,000 tons/yr of tall oil. One U.S. firm alone produced about six million tons of waste lignin from pulping.
Most of the lignin and other organic wastes from pulping are “upgraded” by converting them to energy to raise steam for internal process applications, and in some circumstances generation of electric power for external sales. These wastes are combusted in recovery boilers or recovery furnaces, so-named because they also partially process spent pulping chemicals for eventual recycle after further, post-furnace treatment.
The pulping wastes enter the recovery furnaces as concentrated suspensions of solids in water, i.e. as “waste pulping liquors”. The solids consist of organic residues from pulping and inorganic substances, primarily spent or partially spent pulping chemicals. These processes are the Kraft process, also called alkaline or sulfate pulping, and the sulfite or acid process. Waste pulping liquors from the Kraft and sulfite processes, respectively, typically may contain about 65 to 70 wt %, and 55 wt % solids in water, and are denoted “black liquors” and “brown” (or “red”) liquors.
In the recovery furnaces, waste liquors and their constituents undergo various physical and chemical transformations. It is an instructive approximation to conceptualize a recovery furnace as a process vessel divided into zones dominated by (further) drying and concentration of the waste liquor, devolatilization (pyrolysis) of organics, gasification of organics-derived char and of inorganics (e.g., 2NaOH+CO2 - - - >Na2CO3+H2O), chemical reduction of inorganics, and oxidation of organics-derived volatiles and of inorganics (e.g., Na2S+2O2 - - - >Na2SO4). These phenomena collectively cause the breakdown of organics to intermediates and their subsequent combustion to produce energy, as well as the oxidation and reduction steps that regenerate pulping chemicals or their precursors. For example, in the Kraft process Na2S (an actual pulping chemical) and Na2CO3 (a precursor to NaOH another pulping agent) are bottom-tapped from the recovery furnace as a molten mixture (“smelt”). Recovery boilers are the most expensive single capital cost item in a modern Kraft mill, costing about $100 million for a 2500 to 3000 ton per day paper mill (c. 1995 dollars). Safety is an important issue in Kraft recovery boilers, with roughly 1% of these furnaces having at least one accident per year, e.g., smelt-water explosions.
Various patents describe methods for chemical recovery from waste liquors obtained in wood pulp production. U.S. Pat. No. 4,692,209 describes a process where the liquor is fed into the combustion zone (preferably 1000-1300° C.) of a reactor together with a supply of external thermal energy. The vaporized reaction products pass to a cooling zone (preferably 600-900° C.) and a melt or aqueous solution containing inorganic compounds, particularly NaOH, Na2S and small amounts of Na2CO3 are removed via a lower outlet. An H2 and CO gas mixture is withdrawn at an upper outlet. Temperature and O2 potential in the combustion zone are controlled by regulation of the energy supply and optional addition of carbonaceous material and/or O2 containing gas. The liquor is first subjected to low temperature pyrolysis, e.g., at 600-800° C. and the Na2CO3 and reduced solid carbon mixture obtained is fed to the combustion zone to form small amounts of NaOH and Na2CO3. The mixture is scrubbed with gas from the pyrolysis to obtain an aqueous white liquor containing NaOH, NaHS and Na2CO3. Melt-H2O explosions are avoided.
U.S. Pat. No. 4,710,269 describes a method for increasing capacity and improving the chemical recovery process when using a conventional soda recovery boiler for recovering chemicals from a spent sulfate liquor. The sulfate liquor is supplied to a liquor gasifier while external energy independent of combustion is simultaneously supplied. The temperature and oxygen are controlled independently. The gas product thus obtained containing Na, CO and H2 is introduced into a soda recovery boiler and organic constituents are withdrawn primarily as a gas.
U.S. Pat. No. 5,439,557 describes a process for recovering energy and chemicals from a spent liquor which, after thickening to a dry content of 50-90%, is fed into a reaction chamber having a plurality of zones. The liquid phase is converted to a steam phase. Spent liquor is thermally decomposed to form gaseous organic substances and solid and/or molten organic and inorganic substances, which are reduced and oxidized during the thermal decomposition with oxygen or oxygen containing gas being supplied to the reaction chamber in a controlled amount to maintain the reactions, which comprise combustion of organic substances, and a bed of solid and/or molten substances is formed in a lower temperature zone in the reaction chamber. The steps are carried out during exposure to low frequency sound.
In U.S. Pat. No. 4,808,264, a process is described for recovering chemicals and energy from cellulose waste liquors, preferably black kraft liquor. First, the concentrated black liquor is gasified in a pressurized reactor by flash pyrolysis at 700 to 1300° C. (by introducing oxygen or an oxygen containing gas into the reactor), normally 800-900° C., whereby an energy rich gas is produced and wherein the inorganic chemicals of the black liquor are contained in the form of molten suspended droplets, mainly comprising sodium carbonate and sodium sulfide. Then, the gas from the gasification reactor is rapidly cooled through direct contact with water, and with green liquor, which is formed when the molten droplets and hydrogen sulfide are dissolved in the quench liquid. The cooled gas subsequently passes through a scrubber. In the lower section of the scrubber, the gas is washed with green liquor and, in the upper section of the scrubber, the gas is washed with sodium hydroxide (or carbonate) solution and water for complete removal of any remaining sulfur bearing components. Finally, the sulfur and particulate free gas is used as a fuel for generating steam or for production of electric power.
U.S. Pat. No. 4,917,763 describes a method for recovering chemicals from spent liquors while utilizing energy liberated during the process. The spent liquors are gasified and partially disintegrated in a reactor with thermal energy independent of combustion being supplied by a plasma generator simultaneously to the reaction zone. Then, the resultant melt containing mainly sodium sulfide is separated at substantially the temperature prevailing at combustion. The gaseous product thereby obtained is quenched in a quenching and cooling zone to a temperature below 950° C. The product gas contains substantially no sulfur impurities. Alkali compounds in liquid form are also obtained from the quenching and cooling zone.
In U.S. Pat. No. 4,601,786, a method is described for recovery of chemicals from waste liquor from wood pulp processes, primarily black liquor, while utilizing energy liberated. Controlled total vaporization of the pulp waste liquor at high temperature and low oxygen potential is achieved by external supply of energy using a plasma generator. Subsequent condensation and separation of a melt or water solution is obtained which, without causticizing, can be used for the preparation of white liquor. Also, an energy rich gas consisting primarily of carbon monoxide and hydrogen, and mainly free of sulfur, is obtained.
U.S. Pat. No. 4,116,759 describes a method for regenerating pulping or bleaching chemicals from spent liquor containing salts of polybasic organic acids. The liquor is evaporated and then burned so that organic matter will be discharged as carbon dioxide and water, and a carbonate residue is formed. Dissolving the residue in water regenerates the alkaline salts for pulping or bleaching.
In U.S. Pat. No. 4,738,835, a method is described for recovering alkali chemicals from a material containing dissolved inorganic compounds. The material is gasified in a reactor by an external heat source at a temperature over about 1000° C. producing a gas in which the sodium is substantially in a gaseous state. The gas is cooled by contact with an adequate amount of cooled recirculated solid particles in a sublimation chamber of a circulating bed cooler to decrease the temperature of the gas rapidly below the sublimation temperature of the sodium compounds so that sodium compounds are condensed onto the solid particles. The gas and solid particles pass upwardly through a heat exchanger to cool the solid particles and some of the solid particles are removed from the gas.
JP 1006191 A describes a recovery process wherein the liquor is fed at the top of a reaction zone while external heat energy is simultaneously fed independently of the burning. The temperature and oxygen potential in the reaction zone are independently controlled by the controlled heat energy fed and, if necessary, feeding carbon material and/or oxygen containing gas. Thus, all of the alkali and sulfur are separated from the gaseous phase and combined to the fusion phase which is removed from the reactor. The organic portion of the spent liquor is in the gaseous state.
At the 1989 International Chemical Recovery Conference, L. Stigsson, “A New Concept For Kraft Recovery,” pp. 191-194, disclosed the use of plasm generators for supporting and stabilizing the gasification reactions for recovery of chemicals and energy from kraft waste liquors.
As reported by R. Grant, Pulp & Paper International, Vol. 36, No. 2, page 53 (3) (Feb. 1994), the trend to burning higher black liquor solids continues. Available processes allow more efficient burning of black liquor in the recovery boiler at up to 90% solids reduction efficiency or better and sulfur dioxide emissions are significantly reduced. Gasification is seen as the key to unlocking incremental recovery capacity for mills where the recovery boiler is the bottleneck. Gasification turns black liquor into two separate streams, one (from the organic components) that can be burnt in a gas turbine or elsewhere, and another (from the inorganic components) that provides the green liquor.
However, none of the reported processes use a non-combustion conversion process for obtaining higher value compounds from the organic materials in the spent pulp waste liquors. Further, none of the processes convert other sources of biomass to higher value compounds. Thus, there still exists a need for a commercial process to convert biomass to higher value fuels and chemicals.